As the dimensions of microelectronic devices continue to shrink, and device density continues to increase, the metrology requirements for process development, monitoring, and control continue to tighten accordingly. The accuracy of parameter measurements is becoming increasingly important to optimizing both device performance and chip yield. In order to obtain an accurate and robust monitoring solution, the measurement sensitivity of metrology tools therefore must continue to improve. Measurements can be made at various points in the fabrication process in order to ensure that parameters such as dimensions, profiles, and depths are maintained within specification. In the manufacturing of modern integrated circuits (IC), for example, important parameters of the semiconductor structure can be monitored after each consecutive fabrication step to ensure high quality of the final IC product. One of these parameters is the so-called critical dimension (“CD”). The CD typically refers to the minimum line width that can be fabricated for a microelectronic device. Presently, the CD of a single line feature is too small to be measured optically.
One current conventional metrology technique to monitoring and/or controlling the CD utilizes top-down scanning electron microscopy (CD-SEM), which at best measures an apparent width of a feature or structure. The CD-SEM monitors process excursion by measuring changes in the CD parameter. Single CD-SEM measurements are not enough to control these processes, however. In order to establish a correlation between profile parameters, i.e. CD, side-wall angle, and side-wall shape, as well as other parameters of the semiconductor film stack, such as may include thickness, dispersion of the patterned layers, and/or underlying layers, a combination of several different techniques performing a variety of measurements is required. One existing combination includes a CD-SEM system and an optical thin film metrology system. Measurements using this combination are very time-consuming, and require different test structures and destructive cross-sectional analyses of the wafers.
Another existing metrology approach used for microelectronic devices such as integrated circuits includes a spectroscopic, diffraction-based approach. Such an approach can be preferred over techniques such atomic force microscopy (AFM) because the approach is rapid and non-destructive, and can be preferred over techniques such as CD-SEM due to the relatively inexpensive cost. In a diffraction-based approach, a model of the feature to be measured is constructed, based on a number of variable parameters. This model then can be compared with the actual, measured diffraction data. The parameters of the model can be adjusted until the correlation between the model and the data reaches an acceptable amount. When creating and using a model for such a profile, a regression algorithm capable of determining the profile using spectral intensity data can be used such as described, for example, in U.S. Pat. No. 5,963,329, hereby incorporated herein by reference. A downside to such an approach is the potential complexity of such adjustments. For instance, to measure a line width it is first necessary to define each edge location of the line, through use of a model developed for edge detection. In many situations the number of variables needed for such a model, which often needs to include variables for underlying layers as well, is large enough to effectively prohibit the creation and use of such a model, let alone the creation and use of a library of such models necessary for the various feature types. Further, changes and drift in the fabrication process can introduce significant measurement error, even to the point where the model is invalid for the device being measured. The above-mentioned techniques also are generally unable to accurately characterize submicron structures buried under a planarized, overlying material layer.